Lead acid batteries were invented in the 1860's and the basic structure of a lead acid battery has changed little since then. However, advancements in materials and manufacturing processes continue to improve the energy density, power density, life (including calendar life and cycle life), and reliability.
Typically, lead acid batteries used in the recreational vehicle (RV) and marine Industries consist of two 6-volt batteries in series, or a single 12-volt battery. These batteries are constructed of several single cells connected in series wherein each cell produces approximately 2.1 volts. A six-volt battery has three single cells which, when fully charged, produce an output voltage of 6.3 volts. A twelve-volt battery has six single cells connected in series, producing a fully charged output voltage of 12.6 volts.
A lead acid battery cell consists of two lead plates: a positive plate covered with a paste of lead dioxide and a negative electrode made of sponge-like lead structure, with an electronically insulating material (separator) positioned between the negative electrode and the positive electrode. The flat lead plates in both electrodes are immersed in a pool of electrolyte consisting of water and sulfuric acid.
After some 150 years, lead acid batteries having improved performance features continue to evolve. Modern research and development work is primarily aimed at maximizing the specific power (watts per kilogram) over designated high rate discharge voltage-current profiles, and maximizing battery life, not only in environmental durability but also in cycle life (number of charge-discharge cycles).
The corrosion (mainly on the positive plate) and sulfation (on both the negative and positive plate) are the two key failure modes of lead acid batteries. The corrosion problem begins to accelerate either as temperatures rise about 20° C. and/or if the battery is left in a discharged state. Two main approaches have been followed to mitigate the effects of the corrosion process: developing more corrosion-resistant lead alloys and improving grid manufacturing processes that reduce the mechanical stresses in the as-manufactured grids. In order to improve the battery service life, battery engineers often design the lead alloy and vary the grid wire cross-sectional area for the purpose of changing the grid thickness and corresponding plate thickness. Thicker grids provide longer life, but usually at the expense of reduced power density and increased cost, weight, and volume.
The sulfation failure mode is associated with the formation of “hard lead sulfate.” When a lead acid battery is left on open circuit stand, or kept in a partially or fully discharged state for a period of time, the lead sulfate formed in the discharge reaction recrystallizes to form larger and thicker lead sulfate crystals commonly referred to as hard lead sulfate. This non-conductive lead sulfate, being large and thick, blocks the conductive path needed for recharging. It is difficult for these crystals to convert back into the active materials in the charged state: lead in the negative electrode and lead dioxide in the positive electrode. Consequently, the battery capacity decays rapidly over time.
Even a well maintained battery will lose some capacity over time due to the continued growth of large and thick lead sulfate crystals that cannot be fully recharged during each recharge cycle. These sulfate crystals, of density 6.287 g/cm3, are also larger in volume by about 37% than the original paste, so they mechanically deform the plate and push material apart. The resulting expansion and deformation of the plates also causes active material to separate from the electrodes with a corresponding loss of performance.
Sulfation is the main problem in recreational applications during battery storage when the season ends. For instance, boats, motorcycles, and snowmobiles lie dormant in their off-use period and, left uncharged, the lead acid battery discharges toward a zero % state-of-charge, resulting in progressive sulfation of the battery. Consequently, the battery cannot be recharged anymore, is irreversibly damaged, and must be replaced.
There are several additional drawbacks commonly associated with conventional lead acid batteries. Due to their inherent design and active material utilization limitations, conventional lead-acid batteries only provide relatively good cycle-life when less than about 80% of the rated capacity is used during each discharge event in an application. Such a battery suffers a significant decrease in the cycle life when 100% of the rated capacity is consumed during a discharge. Many new products now require a significant increase in cycle life. For instance, the batteries in hybrid electric vehicles (HEVs) operate in a High Rate Partial-State-of-Charge (PSoC) condition. Such an application is known to dramatically shorten the cycle life of a typical lead acid battery.
Another drawback of the lead-acid battery is the notion that the recharge time is significantly longer than competitive batteries, such as lithium-ion batteries. It would take from 8 to 16 hours to completely recharge a lead-acid battery used in an electric vehicle. A high charge rate is also essential to the operation of an uninterrupted power supply (UPS).
Still another drawback of the lead acid battery is the low specific energy density. For a 2.0 volt cell, the theoretical specific energy is approximately 167 watt-hours per kilogram of reactants (or 167 Wh/kg based on the total weight of reactants). However, a lead-acid cell in practice gives only 30-40 watt-hours per kilogram of battery cell (or 30-40 Wh/kg based on the total cell weight), due to the presence of the mass of non-active materials or components that add weights but not charge storage capacity to the cell. These non-active materials or components include water, grids and/or current collectors, flock, additives (e.g. an “expander” component), a separator, and battery casing, etc. Furthermore, in actual uses of the battery, some portion of the active materials may not be able to contribute to charge storage.
There is a need to increase the active material proportion relative to the non-active portion and to increase the active material utilization rate (i.e. by reducing or eliminating the non-contributing portion of the active materials).
Conventional processes for producing electrode plates for lead-acid batteries generally include mixing, curing and drying operations. In these operations, the active materials in the battery paste undergo chemical and physical changes that are needed for establishing the chemical and physical structure and resulting mechanical strength of the electrode plate. In a typical procedure, materials are added to paste mixing machines in the order of lead oxide, flock, water and sulfuric acid, which are then mixed, dispersed, and homogenized to a paste consistency.
The flock component is a fibrous material, usually composed of polyester, nylon or acrylic fibers, which is added optionally to the paste to increase the mechanical strength of the pasted plate. An “expander” component consisting of a mixture of barium sulfate, carbon black and lignosulfonate may be added to the negative electrode paste to improve the performance and cycle lifetime of the battery. During mixing, chemical reactions take place in the paste, producing basic lead sulfates; e.g. tribasic lead sulfate. The final paste composition is a mixture of basic lead sulfates, unreacted lead monoxide, and residual free lead particles.
Pasting is the process of making a lead acid battery plate. This paste is dispersed into a pasting machine, which applies the paste to a grid structure composed of a lead alloy. The pasted plates are generally surface dried in a tunnel dryer and then either stacked in columns or placed on racks. The stacked or racked plates are then placed in curing chambers. During the entire pasting and curing operation, the paste must have sufficient mechanical strength to avoid micro-crack formation, which otherwise could increase internal electrical resistance. A high internal electrical resistance can limit rates of discharge and charging as well as result in localized heating during charging/discharging and increased chemical degradation of the active materials.
Some efforts have been made to reduce the high impedance of the battery and to accelerate the formation step (first charging step). These include adding carbon black to the paste, which requires adding surfactants in order to properly disperse the carbon black. Unfortunately, surfactants can create higher internal impedance. Additionally, regions of high impedance are present due to the non-homogeneous contact resistance of the powders and, consequently, an overvoltage during battery charging is often applied to overcome this high local impedance. The overvoltage in turn results in electrolysis of water, generating oxygen at the cathode which then rapidly degrades the carbon black. Thus, it is highly desirable to have a means to lower impedance in lead-acid batteries that can avoid overvoltage requirements for charging. A need also exists for a longer lasting conducting additive than carbon black for the electrode of a lead acid battery.
In summary, a need exists for a new type of lead acid battery that exhibits a higher energy density, higher power density, and a more stable or longer cycle life.